Semiconductor power converters employing power semiconductor devices have been widely used so far. Since heat release of the power semiconductor device may cause malfunction or fracture of the semiconductor power converter, the semiconductor power converter is used by cooling the power semiconductor device. In order to cool the power semiconductor device, a forced cooling method is usually adopted in which one of gaseous and liquid cooling fluids such as air, water, fluorocarbon and insulating oil is caused to flow according to scale of the semiconductor power converter and magnitude of heat release value.
This cooling method has been adopted to such an extent that cooling fluid is caused to flow at a predetermined constant quantity or rate determined for a maximum heat release value of the semiconductor device by capability of a fan, a pump, a cooler or the like in order to simplify structure and upgrade economy of the semiconductor power converter as much as possible or that even if control is performed, on-off control of cooling power at the utmost is performed for the purpose of saving the cooling power.
For example, an arrangement of a cooling apparatus of a semiconductor power converter disclosed in Japanese Patent Laid-Open Publication No. 4-130698 (1992) is shown in FIG. 13. In order to explain operational characteristics of the cooling apparatus, FIG. 14 shows temperature changes of a semiconductor device. In FIG. 13, a multiplier 201 multiplies a detected current of the semiconductor power converter (not shown) by an intrinsic constant of the semiconductor power converter so as to obtain a value corresponding to heat release of the semiconductor device. An integrator 202 integrates with respect to time a value obtained by subtracting the value corresponding to heat release of the semiconductor device from the multiplier 201, from a set heat dissipation constant of a setter 203 so as to obtain a value corresponding to a temperature of the semiconductor device. A comparator 204 compares a set temperature of a setter 205 and an output of the integrator 202 with each other so as to obtain a control output for effecting changeover between operation and stop of a cooling fan (not shown).
FIG. 14 shows that the semiconductor power converter is turned on and off alternately several times before temperature of the semiconductor device reaches a set value and the cooling fan is operated for the first time at a time t0. Namely, useless cooling power during this period is saved. In short, since the cooling fan is operated only when there is a risk that the semiconductor device may be fractured thermally, useless operation of the cooling fan is lessened.
However, in the above described cooling method in which cooling capability is kept constant or is subjected to on-off control, it is inevitable that temperature of the semiconductor device fluctuates due to variations of output of the semiconductor device as shown in FIG. 14 or although not specifically shown in FIG. 14, temperature variations of the semiconductor device (referred to also as a "heat cycle", hereinafter) occur in response to turning on and off of the cooling fan. In some cases, the heat cycle adversely affects the semiconductor device more than operating temperature so as to deteriorate the semiconductor device and reduce service life of the semiconductor device, thereby resulting in rise of its failure rate.
Especially, in case a load is a variable speed motor in application of the power converter, such a problem arises that since acceleration and deceleration of the variable speed motor, i.e., increase and reduction of current to the semiconductor device are repeated frequently, so that a severe heat cycle is applied to the semiconductor device even in a constant state of the cooling fan and thus, temperature of the semiconductor device fluctuates frequently, thereby resulting in drop of reliability of the power converter.
The conventional cooling method in which cooling capability is kept constant or is subjected to on-off control has the disadvantage that temperature of the semiconductor device undergoes the heat cycle as described above but has another drawback as follows. In most cases, capability for cooling the semiconductor device is set at a constant value in accordance with a maximum output of the semiconductor power converter. The maximum output referred to above represents one yielded for a period of not less than dozens of seconds (referred to as a "rated output") and heat release due to overcurrent flowing for a duration shorter than the above period is not cooled. This is because if cooling capability controlled to be constant is raised in accordance with the short-time overload rating, cooling capability to be seldom used should be secured at all times, which is quite wasteful.
As a result, for short-time overcurrent, rise of temperature of the semiconductor device is determined by thermal capacity of the semiconductor device itself and thermal capacity of a heat sink (referred to also as a "heat dissipation fin" or a "heat exchanger") to which the semiconductor device is attached.
However, for the purpose of raising reliability of the system, there is always a demand for setting short-time overcurrent capability of the power converter to as high a level as possible. On the other hand, the short-time thermal capacity of the heat sink is restricted by heat transfer rate of material of the heat sink. Therefore, such a problem is posed that a limit is reached in raising the short-time overcurrent capability of the power converter in comparison with the continuous rating.
As one example in which the above mentioned problem affects a concrete configuration of the system, a case of an applied apparatus in which a semiconductor power converter is applied to a DC power transmission system is described. In case a DC power transmission system, for example, is used by connecting a plurality of converters to the system and a fault happens in which rise of fault current is slightly gentle, i.e., for several seconds as in a ground fault of power transmission lines in the DC system, only a power transmission line associated with the ground fault is disconnected by using a high-speed circuit breaker and the circuit breaker is reclosed after recovery of the ground fault. This is described with reference to FIG. 15 showing an arrangement of a conventional DC power transmission system. FIG. 15 illustrates a typical DC power transmission system similar to that described in a book entitled "Electrical Engineering Handbook" edited by the Institute of Electrical Engineers of Japan. In FIG. 15, "1a" and "2b" denote different AC power systems, "2a" to "2d" denote converter transformers, "Da" and "Db" denote power converters used exclusively for rectification and formed by diodes, "3c" and "3d" denote separately excited power converters for inverters, which are formed by semiconductor devices, "Cba" to "CBh" denote DC circuit breakers, "6a" and "6b" denote neutral grounds and "7a" to "7d" denote DC power transmission lines.
Ordinary operation of the DC power transmission system is performed in a state where the DC circuit breakers CBa to CBh are closed. AC power supplied from the AC system 1a via the converter transformer 2a is rectified by the power converter Da used exclusively for rectification and is transmitted as DC power by the DC power transmission lines 7a to 7d. Then, the DC power is again converted to AC by the separately excited power converter 3c and is transmitted to the AC power system 1b through the transformer 2c. In the foregoing, only an upper half portion of FIG. 15 is described but the same applies to a lower half portion of FIG. 15. If a ground fault LG occurs in the DC power transmission line 7b, both of the DC circuit breakers CBb and CBf are interrupted promptly before its current fractures the power converter. Thus, the current of the DC power transmission line 7b is commutated to the DC power transmission line 7a temporarily and the DC circuit breakers are reclosed upon recovery of the ground fault so as to reinstate the DC power transmission system to ordinary operation.
By operating the DC power transmission system as described above, period during which transmission power drops is minimized. However, since it is extremely difficult to manufacture a extra-high voltage DC circuit breaker designed for the purpose of high-speed interruption, such disadvantages are incurred that the system becomes expensive and transmission voltage cannot be set high.
As a method of restraining ground fault current without using the DC circuit breaker, impedance grounding (capacitor grounding) is, needless to say, is known. Detailed description of this method is abbreviated here. When a ground fault occurs in case this method is adopted, voltage to ground at a side of a power transmission line, which is not subjected to the ground fault rises to twice an ordinary value, so that dielectric strength to ground of the DC line should be set at twice that required for ordinary power transmission. Therefore, since such an essential merit is lost that cost for constructing a power transmission line for DC power transmission is more inexpensive than that for AC power transmission, this method is seldom used for extra-high voltage DC power transmission.
As described above, the conventional semiconductor power converters have the drawbacks that the power semiconductor device is readily subjected to the heat cycle and the short-time overcurrent capability of the power semiconductor device is not so high as to be satisfactory.
Meanwhile, in the case of an applied apparatus in which the conventional semiconductor power converter is applied to a DC power transmission system, such an inconvenience is incurred that in order to prevent drop of electric energy to be transmitted in the system at the time of a ground fault, the high-speed circuit breakers for two circuits and the power transmission lines for two circuits are required to be provided, which is expensive.